Redox-switchable materials

ABSTRACT

The invention relates to redox-switchable material comprising a redox-active moiety adsorbed, bonded or both, to a semiconductor material. Among the preferred redox-active moieties disclosed herein are ferrocenes, acridines, and quinones though any such moiety may be employed. The redox-switchable material of this invention may be used to selectively remove one or more selected solutes from an aqueous solution wherein adsorption/complexation of the solute is influenced by the oxidation state of the redox-active moiety. In an alternative embodiment, inclusion moieties that are covalently bound to the redox-active moiety are employed to achieve selective complexation of the desired solute. Other possible applications of the disclosed materials are photoerasable writing media, electrochromic or photochromic materials, catalysis, and solar energy storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application takes priority under 35 U.S.C. 119(e) from U.S.provisional application 60/344,107, filed Dec. 28, 2001, which isincorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates generally to materials that comprise aredox-active moiety and which are selectively reversibly switchablebetween a reduced state and typically one oxidized state (i.e.,redox-switchable). The redox state of the material may be switchedchemically (e.g., oxidizing or reducing agents), electrochemically(e.g., by contact with electrodes, or use of redox-active reagents)and/or photochemically. Materials of interest include solid materials inwhich a redox-active moiety is adsorbed, bonded or both to a surface ofthe solid so that switching of the redox state of the redox-activemoiety affects the properties of that surface, e.g. charge, and/or oraffects adsorption or bonding to that surface. Of particular interestare solids incorporating a photoactive semiconductor, preferably aphotoactive oxide semiconductor, and a redox-active moiety from whichelectron-hole pairs can be generated on irradiation to switch redoxstate.

Redox switchable materials in general have application as redox switches(e.g., selectively changing the redox state or charge of anenvironment), in sensors and other analytic devices or methods, forselective removal, release or transport of ions, and as catalysts. Suchmaterials may also find application in the control of self-assemblyrelated to molecular devices (Muraoka, 2002 and references therein).

A material whose redox or charge state can be “switched” by externaltriggers such as light, electricity, or chemical potential has morespecific application for use in extraction of solutes from solution; inelectrochromic materials, whose optical properties (e.g., color,reflectivity, transparency) change on application of an electric field;in reusable writing media or data storage applications; and inelectrocatalysis in which electricity is employed to drive chemicalreactions, e.g. to carry out more efficient syntheses of valuableproducts or to store electricity by making fuel. Photo redox-switchablematerials can additionally be employed generally in solar energyapplications for storage of light energy in chemical forms for makingfuel.

The extraction of species from solution (herein referred to generally assolutes) by adsorption to a solid is well known, as exemplified by theuse of ion-exchange resins and zeolites. Furthermore, the specific“recognition” of particular solutes by other molecules, which bind tothe solutes preferentially due to complementary fitting of shape,charge, polarizability, etc. has been the subject of extensive researchover the last few decades. Macrocyclic compounds, such as crown ethersand cryptands, among others, are examples of compounds that are nowbeing applied as selective adsorbers or binding agents to the selectiveextraction of solutes from aqueous solution (e.g., Izatt et al., 1991,1996; Hankins et al., 1996).

These extraction methods, however, suffer from an “elution problem”.Once the adsorber or binding agent is saturated with the extractedsolute, release or removal of the solute from the adsorber or bindingagent to recover the solute and/or reuse the adsorber or binding agentcan be difficult and expensive. This is particularly true whenadsorption or binding is highly selectivity. The elution of the solute,and concomittant regeneration of the adsorber, is typically carried outunder extreme chemical conditions. In commercial gold hydrometallurgy,for example, aurocyanate adsorbed onto activated carbon must be removedby elution with strong basic solution. Similarly, the release of heavymetals that have been extracted by crown ethers tethered to silicarequires elution in strong acid (e.g., ˜1 M HCl; Izatt et al., 1991).Even the regeneration of the ion-exchange resin in an ordinary householdwater-softener requires flushing with concentrated NaCl brine. Releaseof solute and regeneration of the adsorber or binding agent can beexpensive and/or wasteful in application of energy or materials. Inapplications to water purification, the result of use of such selectiveadsorbers or binding agents is often the generation of a greater volumeof aqueous waste than the volume of water purified. While such costsand/or waste can be tolerated in particular applications, from a globalperspective such purification technologies hinder rather than help thegoal of water purification.

Materials in which adsorption and/or binding affinity can be selectivelychanged would provide improved adsorber and or binding agents.Redox-switchable materials which also incorporate a selective complexingor binding moiety and in which the complexing or binding affinity isaffected by redox state would provide selectively switchable adsorber orbinding agents useful for overcoming the elution problem discussedabove.

Much research has been described on homogeneous, solution-based systemsinvolving redox-activated binding, e.g., involving functionalizedferrocenes. See, for example, Allgeier, 1997; Beer, 1998; Beer, 1996;Beer, 1995; Beer, 1994; Beer, 1993; Beer, Chem. Commun., 1993; Chen,1995; De Santis, 1992; Hall, 1997; Hall, 1993; Kaifer, 1996; Lloris,1998; Plenio, 1997; Plenio, Chemische Berichte, 1997; Plenio, 1995; andSu, 1999.

Certain redox-switchable materials exhibit binding affinity that is afunction of the redox state. For example, the “ferrocene cryptand”molecule 1(1,1′-[(1,7-dioxa-4,10-diazacyclododecane-4,10-diyl)diethoxy]-3,3′,4,4′-tetraphenylferrocene)displays selective, redox-switchable binding. The ferrocene moietyundergoes reversible one-electron oxidation, while the cryptand moiety(the nitrogen- and oxygen-containing ring) binds selectively to cationsthat fit within the ring. This binding is disrupted when the ferroceneis switched into the oxidized state, because of electrostatic repulsionbetween the bound cation and the positive charge on the Fe atom.

The applications that have been proposed for these homogeneous systemsare in sensors for detecting small quantities of solutes, in whichbinding to the ferrocene would cause a measurable change in potential.Redox-active species are not adsorbed or bonded to solid surfaces.Neither oxidation nor reduction in these systems is reported to belight- or electrically driven. Homogeneous, solution-basedredox-activated binding systems have been reported for use in extractionor separation, for example Clark, 1999; Clark, 1996; Strauss, 1999; andTendero, 1996. For example, Saito, 1986 report the use of a ferrocenefunctionalized crown ether for electrochemical ion transport. Redoxswitching is done chemically, not photochemically or electrochemically.Chambliss, 1998 report the use of modified ferrocenes sorbed (nocovalent bonding was reported) to silica for recyclable anion-exchangematerials.

The attachment of redox-active moieties, such as modified ferrocenes, toelectrode surfaces have been reported. See: Albagli, 1993; Anicet, 1998;Audebert, 1996; Blonder, 1996, Bruening, 1997; Bu, 1995; Chao, 1983;Chen, 1994; Ching, 1995; Ching, 1994; Di Gleria, 1992; Hale, 1991;Moutet, 1996; Schuhmann, 1991; and Wang, 1995. Most of these reportshave been directed toward the detection and analysis of specificsolutes, particularly for biological applications where very smallamounts of solutes are involved. Release of solutes from electrodes insuch applications is not a major concern. The ferrocene/ferriceniumcouple has also been widely used for an electrochemical referencestandard e.g., Bashkin, 1990.

In specific embodiments, this invention relates to materials in whichchanges in adsorption or binding affinity are photo-induced.Specifically, the invention relates to redox-switchable materials inwhich photo-induced redox-switching is mediated by a photoactivesemiconductor.

A number of systems involving photoswitchable binding of a solutespecies have been described. These are not semiconductor based, bututilize photoactive organic molecules of various sorts in which themechanism of photoactivation (e.g., photoisomerization, photocleaving)is very different. Homogenous systems involving photoactive species insolution not affixed to a surface have been reported. Reports includephotoactivated binding to macrocyclic compounds including crown ethersand cryptands. See: Akabori, 1995; Al'fimov, 1991; Barrett, 1995; Blank,1981; Fuerstner, 1996; Irie, 1985; Kimura, 1997; Marquis, 1998; Martin,1996; Shinkai, 1996; Shinkai, 1987; Shinkai, 1982; Shinkai, 1983;Stauffer, 1997; and Tucker, 1997. Insofar as potential applications havedescribed they are generally directed toward sensing. Research has alsofocused on photosystems that are models of biological systems.

Systems involving photoswitchable binding of a solute species directedtoward extraction, separation or ion transport have been reported, butthey relate to homogeneous, solution-based systems. See: Ameerunisha,1995; Effing, 1995; Kimura, 1996, Kimura, 1994; Shinkai, 1981; Shinkai,1982; Takeshita, 1998; and Winkler, 1989. These reports do not involveredox-active moieties attached to a surface.

Photoswitching of membrane permeability has been reported. See: Anzai,1994; Aoyama, 1990; Fujinami, 1993; Hauenstein, 1990; Kumano, 1983;Okahata, 1984; Schultz, 1977. These applications are directed towardextraction or separation, but they do not involve affixing selective,switchable binding moieties to a surface. Instead, they involve mediatedsolution transport through a membrane. They also are notsemiconductor-based.

Semiconductor-mediated photoreduction of species out of aqueoussolution, particularly metal ions, is well-known in the art. See, forexample, Borgarello, 1985; Borgarello, 1986; Brown, 1985;Chenthamarakshan, 1999; Curran, 1985; Eliet, 1998; Herrmann, 1988;Jacobs, 1989; Serpone, 1987; and Tanaka, 1986. Photoreduction of metalions onto semiconductor surfaces has been proposed as technique forextraction of metal from solution (e.g., Borgarello, 1985; Borgarello,1986; and Herrmann, 1988). A major obstacle in the practical applicationof such methods has been economical removal of the metal andregeneration of the semiconductor surface. Photoreduction has also beenemployed for photodepositing metals, e.g., in patterns ontosemiconductor substrates, and is one technique for so-called“electroless deposition”. This technique has been applied wheredeposition is intended to be permanent.

Ionic dyes have also been photoreduced in colloidal semiconductorsuspensions. See, for example, Brown, 1984; Yoneyama, 1972; Gopidas,1989; and Vinodgopal, 1994. However, none of the systems described isswitchable, and none involves a redox-active moiety adsorbed or bound toa solid.

One semiconductor-based light-driven switchable system for dissolutionof metal ions has been described which is based on the reversiblephotoreduction of aqueous cupric ion (Cu++) to an insoluble cuprous(Cu+) complex on the semiconductor. See: U.S. Pat. No. 5,332,508;Foster, 1993 and Foster, 1995. The Cu+ complex reoxidizes on exposure toair, with return of Cu++ to solution. Photoreduction apparently must becarried out in the absence of oxygen. Further, an organic co-solvent isapparently required as a “hole scavenger.” Photoreduction does not occurin simple aqueous solution, evidently because the co-solvent acts as achelating agent for Cu+. Organic contaminants or added organics intreated solutions are photooxidized. This system does not employ aredox-active moiety adsorbed or bound the semiconductor.

Redox-switched extraction by electrochemically controlled reversibleintercalation and de-intercalation into redox-active crystals onfunctionalized electrode surfaces has been reported for lithium ionrecovery (U.S. Pat. No. 5,198,081 and Kanoh, 1993) and Cesium ionseparation (Lilga, 1999). These applications do not involve surfaceimmobilization of particular redox-active binding agents and are neverphotodriven. Photodriven insertion of dissolved species into crystallattices has been reported See, for example, Betz, 1985; Betz, 1984;Betz, J. Appl. Electrochem., 1984; Tributsch, 1980; and Tributsch, 1983.These reports do not involve surface binding of redox-active moietiesand protons, rather than metal ions, are the intercalating species.

Redox-switchable materials also have potential application inelectrosorption; data storage; and light energy storage.“Electrosorption”, the extraction of ions from aqueous solution byelectric fields, is conventionally accomplished by electrostaticadsorption of counterions to electrodes surfaces (Johnson, 1971). Recentinterest focuses on high-surface-area electrodes such as carbon aerogels(Farmer, 1996). Redox-active electrodes have been mentioned only in thecontext of “pseudocapacitance” for boosting the capacity of so-called“ultracapacitors” (Conway, 1997). Redox-swichable materials haveapparently not as yet been applied in such applications.

The use of photoactive molecules for bitwise storage in computermemories have been reported. See, for example, Feringa, 1993;Tsivgoulis, 1995; and Willner, 1997.

Solar energy is expected to become a major primary energy source in thenext few decades. Its disadvantages are well known, as it is bothdiffuse and intermittent. An attractive approach to solving theseproblems is the one already taken by plants: trapping the energy ofsunlight into chemical bonds. Such “artificial photosynthesis”, the useof sunlight to produce fuel, has a literature stretching back to theearly 1970s (e.g., Fukujima & Honda, 1972; Bard & Fox, 1995; Bolton,1996) although it is not yet practical. Photo-switchable redox activematerials in which the energy of the absorbed photons can be trapped ina chemical form can be applied to solar energay storage.

Photoactive semiconductors such as Si and GaAs functionalized withferrocene moieties have been proposed for applications in artificialphotosynthesis. See: Bocarsly, 1980; Bolts, 1978; Bolts, 1979; Bolts,1979, pp. 1378-85; Gronet, 1983; Kashiwagi, 1998; Legg, 1977; andTatistcheff, 1995. The functionalization was employed to shield thesemiconductor from corrosion by the aqueous solution. In contrast tooxide semiconductors, Si and GaAs semiconductors readily photooxidize inwater. Ferrocene is not used as an energy-storage couple in theseapplications.

There is a significant need in the art for redox-switchable materialsfor the various applications noted above and in particular forphoto-driven or activated redox-switchable materials. The presentinvention provides improved solid phase redox-switchable materials.

SUMMARY OF THE INVENTION

The invention provides redox-switchable material which comprises aredox-active moiety adsorbed, bonded or both to a solid surface. In oneaspect the solid is a semiconductor material, for example a photoactiveoxide semiconductor. In another aspect the solid is an electrodesurface.

The redox-switchable material herein functions for selective adsorption,binding or complexing of one or more selected solutes whereinadsorption, binding or complexing of the solute is affected by the redoxstate of the material. The redox-active moiety may exhibit selectiveadsortion, binding or complexing or the redox-active moiety may becovalently bonded to a distinct moiety which exhibits selectiveadsortion, binding or complexing.

The redox-switchable material can comprise a hydrophobic semiconductormaterial. The semiconductor material can be rendered hydrophobic byadsorption, complexation or by ionic, hydrogen or covalent bonding ofhydrophobic species. Preferably the hydrophobic species are notthemselves redox-active under application conditions. Preferredhydrophobic semiconductor materials have hydrocarbyl moieties, e.g.,alkyl, alkenyl, alkynyl or aryl, covalently attached to the surface ofthe semiconductor material. Hydrophobic semiconductor materials includethose in which teh surface is coated with a layer of hydrophobic speciesor a species that is largely hydrophobic, for example a surfactant. Thehydrophobic moieties are preferably bonded to the semiconductor surfacevia O—Si— groups. The semiconductor material can be in the form ofparticles, which typically are powders or finely divided powders, or maybe provided as a surface layer on a substrate, such as glass or ceramicor a plastic or polymer. The semiconductor materials may be provided inpores or channels formed in a substrate material. Oxide semiconductormaterials are preferred.

The redox-active moiety is preferably covalently bonded to thesemiconductor surface and is optionally covalently bonded through alinker group. The linker group can be comprised of any chemcial speciesthat are not oxidized or reduced by irriadiation of the semiconductorand preferably are substantially alkyl linkers. For example, theredox-active moiety can be covalently linked to the semiconductorsurface via a —(CH₂)n- chain, wherein n is 2 or more, preferably is 6 ormore and more preferably is 10 or more. Preferred linkers do not contain—NH— or —O— linkages. Bonding of the linker to the semiconductor surfaceis preferably via —O—Si— bonds. Alkyl linker groups can be fluoroalkylgroups including perfluorinated alkyl groups. Alternatively, theredox-active moiety can provided in a layer on the electrode surface.The layer of the surface may be a layer of polymer or plastic comprisinghte redox-active moiety. The redox-active moiety may be a multimer orpolymer.

Materials of this invention include electrodes to which a redox-activemoiety is adsorbed, complexed or bonded. Preferred electrodes are thosein which the redox-active moiety is covalently bonded to the surface andpreferably the redox-active moiety is bonded to the surface via a linkergroup which preferably may be an alkyl chain or fully or partiallysubstitued alkyl chain whereinteh substituents are not redox-activeunder the application conditions. Allyl linkers may for exampel, besubstitued with one, two or more F's. Alternatively, the redox-activemoiety can provided in a layer on the electrode surface. The layer ofthe electrode surface may be a layer of polymer or plastic comprisingthe redox-active moiety. The redox-active moiety may be a multimer orpolymer.

Any redox-active moieites can be employed in the materials of thisinvention, however, ferrocenes, acridines, and quinones are preferred.The generic classes of redox-active moieties include derivatizedferrocenes, acridines and quinones that are substituted with groups thatdo not deterimentally affect redox-activity or general chemicalstability of the moieties.

The redox-switchable materials of this invention can be a component of areversible redox system which comprises a redox-switchable material incontact with an aqueous solution containing an oxidizing agent, such asoxygen, wherein the redox-active moiety in its oxidized state can bephotoreduced upon irradiation of the semiconductor and in its reducedstate can be oxidized by the oxidizing agent. In exemplary systems,oxygen is present during photoreduction of the redox-active moiety.

Redox-switchable materials of this invention can be employed in methodsfor selective removal, concentration or collections of one or moresselected solutes, most often charged species, from aqueous solutions.Cations or anions can selective removed, concentrated or collected. Theremoval of metal ions or of metal-containing cations from aqueoussolution is a preferred application of the redox-switchablesemiconductor materials and redox-switchable electrodes of thisinvention.

In a specific embodiment, counterions of the adsorbed, complexed orbonded redox-active moieties an be co-adsorbed, co-complexed orco-bonded to the semiconductor and/or electorde surfaces to decrease orneutralize surface charge.

The redox state of the redox-active materials can be electricallyswitched by attachment to an electrode so that its potential may bevaried externally. Redox state may also be switched electrochemically byuse of redox-active reagents. Finally, the surface may be switched bylight by using a photoactive semiconductor as the support. Onillumination with radiation of energy greater than the semiconductor'sband gap, electron-hole pairs are generated near the surface of thesemiconductor. These photogenerated holes and electrons are extremelyeffective “reagents” for carrying out oxidations and reductions,respectively. In particular, under the appropriate conditions one orboth can cause redox reactions in surface-bound moieties.

The redox moieties may also “switch back” spontaneously under theappropriate conditions, for example through reaction with atmosphericoxygen. Such spontaneous switching is also of interest in a number ofpotential applications, as described below.

In systems in which switchable solute binding is driven by light.Elution of the adsorbed species will occur merely by changing theillumination. No additional reagents are required, and no new wastewateris generated. Instead, the adsorbed solutes can be concentrated into asmaller volume of solution. Light can be used to drive a redox-activesystem by using a semiconductor as the substrate. Absorption of photonshaving energy greater than the semiconductor's band gap generateselectron-hole pairs, which are highly active “reagents” for causingredox reactions. The holes are powerful oxidizing agents, while theenergetically promoted electrons are strong reducing agents.

In another embodiment, materials and methods for use inelectrically-switched extraction of selected solutes from aqueoussolutions are provided. Redox-switchable materials described herein canbe employed to functionalize electrode surfaces to enhance efficiency ofsolute adsorption and/or release.

The materials herein can be employed for extraction and separationapplications, in catalysis, in switchable extraction, particularly viaphoto-driven switching, in pollution control, water purification,desalination and hydrometallurgy. In addition, electrosorption byredox-active surfaces can employ such materials.

Switchable redox materials, inluding those of this ivention, can beapplied to the generation of reusable writing or data storage media forgeneral information science applications, for storage of light energy inchemical forms, as in “artificial photosynthesis” for making fuel fromsolar energy, generally for making electrochromic materials, whoseoptical properties (e.g., color, reflectivity, transparency) change onapplication of an electric field; and in electrocatalysis which useselectricity to drive chemical reactions, e.g. to carry out moreefficient syntheses of valuable products or to store electricity bymaking fuel.

The invention is further illustrated by the drawing, and the detaileddescription and examples that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a self-reversing chemionic switchemploying ODS-TiO₂, a hydrophobic semiconductor functionalized withoctadecyltrichlorosilane). Holes generated by near-UV illumination ofthe semiconductor oxidize solvent (H₂O) to generate hydrogen ions (H⁺),while photogenerated electrons reduce solution ferrocenium (Fc⁺) toferrocene (Fc). Fc dissolves into the hydrophobic layer on tehsemiconductor surface. After storing the system in the dark for severalhours the ferrocene spontaneously reoxidizes by reaction withatmospheric O₂.

FIG. 2 is a schematic illustration of improved electroadsorption methodand apparatus. Electrode surfaces have “latchable” charges due toattached reversible redox species. Fc=ferrocene; Fc⁺=ferricenium;A⁻=tethered anionic group; Cl⁻=solution chloride; Na⁺=solution sodium.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to materials which are redox-switchable.These materials comprise at least one redox-active species or moietywhich is switchable between a reduced state and at least one oxidizedstate. A redox-active moiety or species contains one or more chemicalfunctions or groups that can be reduced and oxidized, i.e. can acceptelectrons to be reduced and donate or transfer electrons to be oxidized.The term oxidation/reduction state of the redox-active moiety (or othermaterials or species) refers to whether the moiety is oxidized orreduced. For a given redox-active species there is a reduced state andat least one oxidized state and certain redox-active species may havemore than one oxidized state, dependent upon the number of electronstransferred to achieve the oxidized state.

The redox-switchable materials of this invention may in additionfunction for adsorption, complexation or bonding to another chemicalspecies. Most simply, the oxidation/reduction state of the redox-activemoiety can facilitate adsorption of one or more chemical species to ordesorption of one or more already adsorbed chemical species from theredox-switchable material. For example, the oxidation/reduction state ofthe redox-active moiety can result in a change in charged state(positive or negative) of the redox-switchable material which canfacilitate adsorption or desorption of chemical species. Morespecifically, the oxidation/reduction state of the redox-active speciescan affect the charge state of surfaces of the redox-switchable materialto affect adsorption of chemical species on those surfaces. Inparticular, the formation of positive or negative charge on a surfacecan decrease adsorption of a non-polar or hydrophobic species on thesurface. A net decrease in charge on a surface can decrease adsorptionof a polar or ionic species (of opposite charge) on a surface.

Alternatively or additionally, the materials of this invention maycontain a selective complexation moiety which contains one or morechemical functions or groups that can complex, ligate, intercalate orotherwise bond to another chemical species. The term complexation isused generally for bonding or ligating through complexation,intercalation or other bonding (e.g., hydrogen bonding) to the chemicalspecies and is preferably reversible. The complexation is preferablyselective for a given chemical species or class of related chemicalspecies, e.g., selective for a particular species size range, selectivefor a particular ion, etc. A selective complexation moiety can be aligand, including a monodenate, bidentate, tridentate and othermultidentate ligand and it may be a macrocycle, such as a crown ether,cryptand or related species. The redox-active moiety of this inventionmay also be a selective complexation moiety or the redox-active moietymay be covalently bonded to a selective complexation moiety. In specificembodiments, the oxidation/reduction state of the redox-moiety affectsthe complexation or binding affinity of the selective complexationmoiety. For example, the selective complexation moiety may function forcomplexation to a given chemical species when the redox-active moiety isin one oxidation/reduction state and not function for complexation tothat chemical species in another oxidation/reduction state.

The general term solute is used herein for a chemical species which canbe selectively complexed to the selective complexation moiety fromsolution. Solutes include inorganic and organic anions, includingvarious oxides and complex anions containing metal atoms, sulfur,phosphorous and/or halogen atoms and inorganic and organic cations,particularly metal cations and metal-containing cations. Solute anionsand cations can include toxic or radioactive anions and cations whichare beneficially removed to purify aqueous media. Solutes can alsoinclude non-ionic species which may be selectively.

In specific embodiments, a redox-active moiety of this invention isadsorbed to the surface of a solid material or within pores or channelsof a solid material and can affect the charge state of the surface,pores or channels of that solid. The oxidation/reduction state of theadsorbed redox-active moiety can affect adsorption to the surface of thesolid or into the pores or channels of the solid. In a specific example,the oxidation/reduction state of the adsorbed redox-active moietyaffects its own adsorption on the solid, i.e., the redox-active moietyis adsorbed in one oxidation/reduction state and desorbed in anotheroxidation/reduction state. The solution environment with which theredox-switchable material is in contact may additionally affect thecharged state of the surfaces, pores or channels of the solid and mayaffect adsorption on or in the surfaces, pores or channels.

In other specific embodiments, a redox-active moiety of this inventionis covalently bonded to a surface (or within a pore of channel) of asolid and can affect the charge state of the surface, pores or channelsof the solid. The oxidation/reduction state of the adsorbed redox-activemoiety can affect adsorption to the surface of the solid or into thepores or channels of the solid.

In other specific embodiments, a redox-active moiety of this inventionhas a selective complexation function or is bonded to a selectivecomplexation moiety and the redox-active moiety and any bonded selectivecomplexation moiety is bonded to a solid material. Theoxidation/reduction state of the bonded redox-active moiety affectsselective complexation of the redox-active moiety or that of theselective complexation moiety.

In preferred embodiments the solid to which the redox-active moiety isadsorbed or bonded is a semiconductor material, particularly aphotoactive semiconductor material, and more preferably is an oxidesemiconductor material.

Semiconductor materials useful in this invention include photoactivesemiconductors which generate electron-hole pairs on irradiation atappropriate wavelengths. Of particular use is titanium dioxide (TiO₂)and other oxide semiconductors including V₂O₅, ZnO, SnO₂, Fe₂O₃(including α-Fe₂O₃ and γ-Fe₂O₃) In₂O₃, ZrO₂, WO₃, MoO₃ and SiC (which iscovered with a monolayer of SiO₂), and sulfide semiconductors such asZS, CdS, or MoS₂. The semiconductor may also nclude mixed oxides such asilmenites, FeTiO3, or spinels such as FeCrO4, or perovskites such asSrTiO3, or other mixed oxides such as pseudobrookites. vvSemiconductormaterials may exist in different forms which exhibit more or lessactivity for the photoactivity, for example anatase TiO₂, is a preferredoxide semiconductor form.

Semiconductor materials may be doped with metal ions (e.g., Si, Al, Mg,V, Cr, Mn, Fe, Nb, Mo, W or Ru) introduced into their lattice tobeneficially modify their properties, such as absorption orconductivity, for use in this invention. Semiconductors may have metal(transition metals, e.g., Cu or Ni, or other metals, e.g., Rh, Pd, Ag,Pt, Hg) deposited on their surfaces to beneficially modify theirproperties, such as absorption or conductivity, for use in thisinvention.

Semiconductors may be used in different physical forms, as particles,finely divided particles, or embedded, coated, layer or incorporatedinto other materials The semiconductor may be provided as a layer on asubstrate, e.g., a non-semiconductor solid, such as glass, quartz,plastic or a solid polymer.

Semiconductors having high surface area are desirable which may be inthe form of fine powders or of high-surface area solids.

In certain embodiments, hydrophobic semiconductor materials areemployed. Hydrophobic semiconductor materials have an adsorbed, layeredor bonded hydrophobic layer, particularly a hydrocarbyl (componentscontaining only carbon and hydrogen) layer and more particularly ahydrocarbon, e.g., alkyl, layer.

Functionalization of the semiconductor surface may be carried out bynoncovalent attachment, as with surfactants such as fatty acids,phosphonates, or sulfonates. Functionalization may also be carried outby covalent attachment using methods including, but not limited to,reactions of surface hydroxyl groups with chlorosilanes, alkoxysilanes,aminosilanes, or other functionalized silanes; or with reactions withmetal carbonyls; or with photoactivated heterocyclic azides; or by othermeans. By use of different moieties on the functional groups the surfacecan be rendered hydrophobic (e.g., with alkyl moieties), or hydrophilic(e.g., with carboxylate, sulfonate, phosphonate, ammonium, or chargedmoieties), or otherwise be given useful properties.

In other embodiments the solids to which the redox-active materials areadsorbed or bonded are electrodes. Electrode materials include metals,semiconductors, oxide semiconductors, carbon (e.g., porous carbon andcarbon aerogel electrodes).

Exemplary redox-active moieties are ferrocenes, acridines and quinones.

Various means are known in the art, as exemplified in references citedherein, for bonding redox-active moieties, such as ferrocene, to solidsurfaces and particularly to oxide and sulfide semiconductors surfacesand to electrode surfaces.

The redox-switchable materials of this invention are switchable betweenoxidation/reduction states for example by irradiation with light ofselected wavelength (irradiation of a photoactive semiconductor),electrically (employing electrodes), or chemical (use of oxidation orreduction agents).

The invention provides exemplary redox-switchable systems in which theredox-materials of this invention are operational contact with a sourceof electrons for reduction of the redox-active moiety or an electronacceptor to which electrons can be transferred to oxidize theredox-active moiety. The redox-switchable materials may be in contactwith an electrode, or a photoactive semiconductor (with light source),and/or a chemical oxidizing agent or reducing agent. Theredox-switchable materials may be in contact with an environment or amedium, such as a solution, particularly an aqueous solution, whichprovides or removes electrons, or contains one or more oxidizing orreducing agents. The redox-switchable materials may be in contact with amedium that provide oxygen for reoxidation of a redox-active moiety inits reduced state.

Several exemplary switchable, self-reversing systems based on photoredoxreactions driven by semiconductor light absorption are described herein.The terms “self-reversing” is used for those systems in which anoxidizing agent or reducing agent present in the system functions tooxidize or reduce (i.e., to switch back) the redox-active material. Theterm is used in particular to refer to oxidation of the redox-activemoiety by ambient oxygen present in the system after initialphoto-induced reduction of the redox active moiety. The term“spontaneous” as applied to redox reaction herein refers to reactionwhich proceed without application of an external trigger, such asapplication of a voltage, irradiation with light, addition of one ormore redox reagents, addition of an oxidizing agent, addition of areducing agent. As used herein the term “spontaneous” reaction includesoxidation mediated by ambient oxygen levels in the system.

1. Self-Reversing Photodriven Systems: Adsorption of Redox-ActiveSolution Species

a. Ferrocene and Hydrophobic Semiconductor.

Ferrocene(dicyclopentadienyliron(II), (C₅H₅)₂Fe, abbreviated Fc is wellknown to undergo reversible one-electron oxidation to the ferriceniumcation, Fc⁺. Irradiation of a solution of a ferricenium salt in asuspension of a hydrophobic semiconductor, such as TiO2 functionalizedwith octadecyltrichlorosilane as described below, causes quantitativereduction of the ferricenium to ferrocene, with a concomittant decreasein solution pH. Electron-hole pairs are generated in the semiconductor;the holes immediately oxidize solvent molecules:H₂O+2h ⁺=>2H⁺ +½O ₂,leaving the electrons available to reduce solution species:Fc ⁺ +e ⁻ =>×Fc.The overall stoichiometry is:2Fc++H₂O=>2H⁺+2Fc+½O₂.

The change is strikingly apparent visually, as the initial ferriceniumsolution is blue while the suspended semiconductor particles are white.On illumination, the solution becomes colorless due to reduction of theferricenium, while the semiconductor particles become yellow due todissolution of the ferrocene into the hydrophobic surface layer.Ferrocene is extremely hydrophobic and thus is insoluble in water, andhence the hydrophobic layer on the semiconductor extracts it fromaqueous solution. This reaction has been demonstrated both by measuringthe drop in pH (according to the overall equation above) and byrecovering the reduced ferrocene from the hydrophobic layer by solventextraction.

The overall reaction is thermodynamically “uphill” and representsstorage of some of the energy of the photons. Back-reaction of ferrocenewith O₂ is very slow, however, in the absence of catalysts. One usefulcatalyst is H⁺: if the solution is strongly acidic (˜pH 0, e.g. 1 M HClsolution), the overall reaction spontaneously reverses over the courseof a few hours, so long as the solution remains exposed to air (i.e.,oxygen). Addition of a catalytic amount of an Fe³⁺ salt, such as ferricchloride, to the solution catalyzes the back-reaction at an even higherpH. A variety of analogous catalysts can be used.

This system is a spontaneously reversing system. The forward (uphill)direction is driven by light, while the reversion occurs spontaneously,i.e., without an external trigger,—but not immediately—from reactionwith atmospheric oxygen.

It was unexpectedly found that once the ferrocene has been reduced outof solution and incorporated into the hydrophobic layer of thesemiconductor, the holes produced by continued illumination of thesemiconductor did not oxidize this ferrocene. Our initial efforts weredirected toward the use of photogenerated holes to oxidize ferrocene.Without wishing to be bound by any particular explanation, we believe,in hindsight, that the oxidizing potential of the holes generated byirradiation of the semiconductor is so different from that of ferrocenethat there is little quantum mechanical overlap, and hence littletendency for reaction.

This systme is describe in more detail in Muraoka, 2002 which isspecifically inocrporated by reference herein in its entirety for suchdetail. Supporting information that accompanies the journal article andwhich is available from the publisher is also incorporated by referenceherein.

This system provides a chemionic switch, as illustrated in FIG. 1,involving reduction of ferrocenium (off) to ferrocene (on) andreoxidiation to ferrocenium with reduction driven by near-UV light andoxidation by atmospheric oxygen. This system can be readily implementedwith other redox-active moieties, including acridines and quinones.

Exemplary Covalent Attachment of Hydrophobic Layer to Semiconductor

The semiconductor can be rendered hydrophobic in a number of ways. Inthe case of TiO₂, for example, a surface layer of alkyl chains can becovalently bonded by reaction with a hydrophobically substitutedtrichlorosilane, e.g., such as octadecyltrichlorosilane (ODS). Thechlorine atoms react with hydroxyls on the TiO₂ surface to yield acovalent Ti—O—Si bond, while the chlorines between adjacent silicons arehydrolyzed to yield a network of 2-dimensional Si—O—Si links:

where R is a hydrophobic group (most generally a hydrocarbyl groupcontaining only carbon and hydrogen, e.g., alkyl, alkenyl, alkynyl, oraryl groups, preferably hydrocarbyl groups having more than 6 carbonatoms and more preferably more than about 10 carbon atoms). The TiO₂thus becomes coated with a “waxy” surface and is therefore stronglyhydrophobic. This is shown during synthesis: the TiO₂ is first mixedwith dry toluene and agitated in an ultrasonic bath, during which theparticles tend to agglomerate and sink, due to the hydrophilic nature ofthe raw TiO₂ surface. After addition of ODS, however, the particlesdisperse in the nonpolar toluene, implying a change in the initialhydrophilic surface of the TiO₂. This hydrophobically functionalizedTiO₂ is termed “ODS-TiO2”.

Rendering oxide surfaces, including TiO₂, hydrophobic in this way hasbeen described in the literature (e.g., Moses et al., 1978; Murray,1980; Wang et al., 1998). We have also functionalized other oxidesemiconductors such as SnO₂, α-Fe₂O₃, γ-Fe₂O₃, In₂O₃, as well as and SiC(which is covered with a monolayer of SiO₂). WO₃ is not functionalizablein this way because of its extremely low pzc (<0.3), so that surfacehydroxyls are not present. Other linking reagents, e.g., substitutedcobalt carbonyls (Chen et al., 1992) or azides (Harmer, 1991) may alsobe used to establish covalent linkage of a hydrophobic surface moiety toa substrate. The listed references are incorporated by reference hereinto provide specific example methods for generating hydrophobicsemiconductor materials.

b. Adsorbed Anion Switching.

Large relatively hydrophobic anions such as PF₆ ⁻ adsorb onto the ODSlayer of the hydrophobic semiconductor. These anions are expelled, i.e.,photodesorbed, by electrostatic repulsion on illumination of thesemiconductor because of a build-up of negative charge. Suchphotodesorption can also occur with some anionic surfactants on theunderivatized TiO₂ surface near its point-of-zero-charge (pzc). Largeions like PF₆ ⁻ are proxies for toxic or radioactive ions such asradioactive TcO₄ ⁻, whose removal from contaminated water is currently amatter of great interest, or for Au(CN)₂ ⁻, which is basic to goldhydrometallurgy and (ClO₄ ⁻), the removal of low levels of which fromgroundwater is also a matter of increasing interest. These large anionscan be selectively photodesorbed from hydrophobic semiconductormaterials, particularly oxide semiconductor materials, by irradiation ofthe semiconductor material to generate electron-hole pairs. Anions,particularly large anions having four or more atoms, can be selectiveremoved from aqueous solutions by adsoprtion to a semiconductor materialhaving a hydrophobic surface and removal of the semiconductor materialcarrying the adsorbed anions from the aqueous solution. Anions arerelease from the semiconductor material by irradiation of thesemiconductor material in solution to generate negative charge on thesemiconductor surface.

c. Adsorbed Cation Switching.

Under the appropriate circumstances, hydrophobic, e.g., organic,functionalization of the semiconductor is not necessary to exploit thephotoinduced accumulation of electrons on the semiconductor surface. Inaqueous solution, the surface of oxides such as TiO₂ is in generalcharged. There is an intermediate pH at which the number of positive andnegative charges on the surface are equal, so that the overall surfacecharge is neutral. This pH is termed the “point of zero charge” (pzc).For TiO₂ pzc is at about pH 5.6. At pH lower than the pzc of thematerial, exposed oxide ions at the surface will take up protons toyield an overall surface positive charge, whereas at pH higher than thepzc, the surface is deprotonated to yield an anionic surface. Chargedsurfaces attract counterions of the appropriate charge by simpleelectrostatic attraction, and this establishes the well-known “doublelayer” of balancing charges at an electrolyte interface. Hence, at theproper pH UV irradiation of the semiconductor will change the charge ofthe surface so adsorbed species are desorbed due to simple electrostaticrepulsion. Thus, charged species can be selectively adsorbed from thesemiconductor by adjusting the pH to generate an appropriately chargedsurface (opposite in charge from that of the charged species to beadsorbed) and thereafter selectively desorbed by irradiation.

Moreover, literature review indicates that photoaccumulation ofelectrons on oxide semiconductors is a general phenomenon. Other oxidesalso have different pzcs and hence will adsorb differently at differentpHs. Moreover, adsorption of particular solution species may beheightened, or decreased, by nanoscopic details of the oxide surfaces.

Additionally, certain oxide crystal structures have relatively openstructures into which counterions can be intercalated. In particular,tungsten trioxide (WO₃) has a very open structure containing channelsthat can accommodate small ions such as Li⁺. To maintain charge balance,some of the W⁶⁺ in the crystal is reduced to W⁵⁺ on Li⁺ intercalation. Agreat deal of research has been done on electrochemical intercalation ofLi⁺ into WO₃ as a basis for electrochromic materials. In the case ofWO₃, reduction of some W⁶⁺ to W⁵⁺, to maintain charge balance, placeselectrons into the conduction band where they become delocalized toyield typical metallic behavior. Hence the material changes fromtransparent to reflective. Photointercalation of Li⁺ from organicsolvents into WO₃ has also been demonstrated.

Therefore we are investigating whether Li⁺ can be photoextracted fromaqueous solution onto WO₃. This would have an immediate application inhydrometallurgy, because lithium is extracted from natural brines inwhich it occurs at low concentrations against a background of muchlarger concentrations of common ions such as Na, K, and Mg. These largercations, however, would not be expected to intercalate, and so such asystem could have great selectivity for Li. Moreover, WO₃ exists inseveral polymorphs, with metastable polymorphs including much largermolecular-scale voids and channels having been synthesized onlyrecently. To our knowledge these polymorphs have never been investigatedfor their photochemical and photointercalation properties.

WO₃ also has a much lower pzc, ˜0.3, than TiO₂; hence its surface isanionic at all but the very lowest pHs. Finally, WO₃ like TiO₂ isnontoxic.

There are also a number of mixed transition-metal-containing oxides withvery open structures, such as tungsten and titanium phosphates, thatshould be investigated. Like zeolites, these compounds contain channelsand voids that can accommodate larger cations such as Na and Mg. So faras we have been able to determine, however, these compounds have notbeen investigated for their photoactivity. If they are capable ofaccumulating photogenerated electrons, they may be able to intercalatebigger cations, with obvious applications in desalination andpurification.

2. Self-Reversing Photodriven Systems: Adsorbed Surface Species

a. Photoswitching by Cation Adsorption/Surfactant Desorption

Non-covalent attachment of hydrophobic molecules, e.g., by adsorption ofsurfactants such as oleic or other long-chain carboxylic acids (Drelichet al., 1998), hydroxamates (Folkers et al., 1995), hydroxylamines(Marabini & Rinelli, 1982) or phosphonates (e.g., Gao et al., 1996), canalso be used to render a surface hydrophobic. We have demonstratedprototype switching of solute concentrations in semiconductor materialshaving surface adsorbed surfactants.

In the case of oleate (CH₃(CH₂)₇CH═CH(CH₂)₇COO⁻), for example, theconcentration of Ca⁺⁺ in the solution decreases on illumination of thesemiconductor suspension. The photogenerated accumulation of electronson the semiconductor surface causes desorption of oleate due toelectrostatic repulsion. Oleate is then free to bind Ca⁺⁺, for which itis known to have a strong affinity (e.g., Matijevic et al., 1966). Inaddition, oleate binds strongly to TiO₂ at pHs in the vicinity of thepzc, where the surface charge should be most sensitive to photogeneratedchanges.

A portion of the observed decrease in Ca++ concentration in theseexperiments may also result because Ca⁺⁺ is free to coupleelectrostatically with bare (due to oleate desorption),negatively-charged TiO₂ surface. However, we have also preparedsurfactant monolayers of N-hydroxyoctadecanamide (Folkers et al., 1995)and octadecylphosphonatic acid (Gao et al., 1996) on TiO₂ particles, andin these cases no change in Ca⁺⁺ was observed on illumination. Thisindicates that the affinity of the desorbed surfactant anion for thesolute cation is an important factor in the results observed.

b. Photoswitching by Adsorbed Quinone Derivatives

Quinones are a well-known redox system in organic chemistry. Indeed,they are the basis of many biological electron-transfer reactions.Quinones readily and reversibly undergo reduction to the “hydroquinone”form:

where the quinone and hydroquinone are on the left and right sides ofthe equation, respectively, and has the stoichiometry:Q+2e ⁻+2H⁺ =>QH₂.

-   -   quinone (oxidized) hydroquinone (reduced)        In turn, hydroquinones are easily reoxidized to quinones. As        shown above, commonly exposure to air is all that is required:        QH₂+½O₂ =>Q+H₂O.

Many quinones are hydrophobic in both their reduced and oxidized formsand hence insoluble. Napthoquinones (e.g., 2), for example, are quitehydrophobic because of the fused aromatic ring. Such molecules adsorbreadily to a hydrophobic surface, such as ODS-TiO₂, and so provide asimple way to functionalize the surface directly. Moreover, manysubstituted quinones are known to be good chelating agents for metalions. Hence, redox switchable quinone derivatives may be able toreversibly bind solute ions from solution, depending on their redoxstate.

We have tested whether a series of naphthoquinone derivatives wasoxidized by UV-light on ODS-TiO₂. In particular,2-anilino-1,4-naphthoquinone (AnNQ; 3), in a

suspension of TiO₂-ODS, undergoes ready photoreduction to thehydroquinone (4) form on UV illumination (Table 1). The quinone form isa deep red-purple, whereas the hydroquinone form is white; this leads toa striking color change on illumination. In contrast to the ferrocenecase, both forms are also insoluble in water, so they remain on the ODSlayer atop the TiO₂. In the dark the white form reoxidizes withatmospheric O₂ over several hours, returning to the original red-purplecolor. Interestingly, this photoswitching does not occur in the presenceof plain TiO₂, probably because of its hydrophilic nature.

Other quinones have been investigated and several respond similarly(Table 1), though they do not necessarily remain bound to the ODS-TiO₂surface. Whether a given quinone can be photoreduced apparently dependsboth on thermodynamic factors (i.e., the quinone's reduction potential)and on kinetic factors, such as the ease of electron transfer from theunderlying semiconductor.

Binding experiments with quinones: Since AnNQ has a carbonyl oxygen inproximity to a nitrogen, it was expected to bind metals; however, AnNQbinds to metals about equally well in both its oxidized and reducedforms. In contrast, quinolinoquinone (5), is known to bind metals in theoxidized, but not in the reduced form. It also undergoes photoswitchingas indicated the results summarized in Table 1.

Additionally many dyes are quinone-based. Typically a dye is made “fast”by binding a metal ion, which changes it from the colorless “leuco” form(typically the hydroquinone) to the final insoluble colored form (theso-called “lake”), with oxidation occurring on metal binding. Hence themany quinones-based dyes which are known to bind metals can be employedas for switchable binding.

Photoswitching by Adsorbed Acridines

Substituted acridines 6 are known to undergo reversible redox reactionsby addition of a proton opposite the nitrogen in the middle ring, withconsequent loss of aromaticity in the middle ring and of the positivecharge localized on the nitrogen:MeA++2e ⁻+2H⁺ =>MeAH₂.Thus, as with Fc-Fc⁺, an oxidized cationic form is reduced to a neutralspecies. As with the naphthoquinones, however, both the reduced andoxidized forms are hydrophobic, and so can remain largely adsorbed to ahydrophobic substrate such as ODS-TiO₂.

We have demonstrated that this reduction can be driven photochemicallywith ODS-TiO₂, where R=a hydrocarbyl group or substituted hydrocarbylwherein the substitution does not detrimentally affect the stability orredox properties of the molecule. Hydrocarbyl groups include alky (e.g.,methyl, ethyl, propyl, butyl, etc.), alkenyl, alkynyl and aryl groups(e.g., phenyl). The hydrocarbyl may be substituted for example with oneor more halogens, including F, Cl and or Br.

The change is again visible: the oxidized form is yellow whereas thereduced form is colorless. The reaction spontaneously reverses in thedark over several days.

Substituted acridines have been widely studied as a model for theNAD-NADH enzyme system, which is a common biochemical redox mediator. AsNAD is known to catalyze the oxidation of ferrocene, acridine is alsopotentially another catalyst for the ferrocene-based system.

Substituted acridines are also known to bind metals, and as such can beemployed for redox-switchable metal binding.

Redox-Active Species in Hydrophobic Layer

A number of experiments in which a hydrophobic redox-active species isimmobilized in the hydrophobic layer atop the semiconductor by simplehydrophobic interaction have been carried out. These experiments includecases in which the layer is merely an adsorbed surfactant, as well asthose in which it is covalently attached (e.g., as with ODS-TiO₂).

a. Ferrocene Cryptand.

Ferrocene cryptand (1) was immobilized in hydrophobic layers on TiO₂.These layers include the surfactants oleic acid, N-hydroxyoctadecanamideand octadecylphosphonic acid on TiO₂, as well as ODS-TiO₂. We initiallythought that cation switching could be achieved through oxidation of theferricenium moiety by the photogenerated holes. On oxidation the cationwould be expelled through electrostatic repulsion. However, we haveconfirmed the counterintuitive result that the ferrocene cryptand is notoxidized by photogenerated holes, the holes react with the solventinstead. Switching of selective binding agents such as FcCrp canalternatively be achieved by photoreduction, so that take-up of cationswould take place under illumination. Reoxidation by atmospheric O2 inthe dark would then expel the cations.

b. Immobilization of Substituted Ferricenium

Photoreduction of Fc⁺ out of aqueous solution into ODS-TiO₂ wasdescribed above. In an attempt to affix ferricenium to the TiO₂ surface,a substituted ferrocene was synthesized having an 18-carbon alkylsidechain (“C₁₈Fc”). By making the whole molecule considerably morehydrophobic, even in the oxidized state, this indeed rendered theferricenium form insoluble in water. Since the length of alkyl chain issame as that of hydrophobic layer on the ODS-TiO₂, C₁₈Fc could“interfinger” with that octadecyl chain by hydrophobic interaction toyield a surface functionalized with ferrocene. However, although neutralC₁₈Fc evidently binds to ODS-TiO₂, as evidenced by the ODS-TiO₂particles acquiring a pale yellow tinge, this layer is apparently notordered. Addition of benzoquinone yielded no color change, indicatingthat the Fc was not oxidized. Oxidation would not be expected to occurif on the average Fc groups were not exposed at the surface, as wouldhappen if the C₁₈Fc layer formed a disordered tangle atop the ODS layer.However, if the C₁₈Fc was first oxidized to C₁₈Fc⁺ with benzoquinone inacidic ethanolic solution, an ordered layer evidently forms on theODS-TiO2. The ODS-TiO₂ changes to pale blue, indicating adsorption ofthe C18Fc, and it turns pale yellow when irradiated with UV light,indicating photoreduction of the surface ferrocenes. Presumably when theC₁₈Fc is oxidized it acts as a surfactant, with a polar headgroup andhydrophobic tail. Such a configuration is much more likely to lead to alarge degree of spontaneous organization than a completely hydrophobicmolecule.

Unfortunately, the substituted ferrocene is also now considerably moredifficult to oxidize with atmospheric O₂, probably because thenow-adjacent cationic sites impose an additional electrostatic energybarrier. The increased hydrophobicity may also impose a kinetic barrierto electron transfer from the aqueous solution. However, catalysts canbe employed to overcome this effect.

4. Covalent Tethering of Redox-Active Species to Semiconductor Surfaces

Redox-active surfaces can be generated by covalently binding theredox-active moieties onto the surfaces. Such surfaces are expected tobe considerably more stable over a considerably wider range of chemicalconditions (e.g., pH), than surfaces based on surfactant adsorptionand/or hydrophobic interactions.

a. Covalent Attachment of Substituted Ferrocenes

Several methods of attaching ferrocenes to the semiconductor surfacehave been used. These methods were carried out in two steps: (1)covalent attachment of an organic moiety bearing a functional group tothe semiconductor surface; and (2) linking a substituted ferrocene tothat functional group.

Step (1) is merely analogous to the covalent binding of the hydrophobiclayer to semiconductors as already described. Hydrolysis of asubstituted silyl group establishes a covalent link to the semiconductorsurface; however, instead of being an unsubstituted hydrocarbon, the“tail” off the surface now contains a functional group that can befurther reacted to link another molecule. The term linker is used herefor moiety that links redox-active moiety to a surface.

Aminopropyltriethoxysilane, (EtO)₃Si(CH₂)₃NH₂, andchloropropyltrimethoxysilane, (MeO)₃Si(CH₂)₃Cl were employed as linkers.TiO₂ covalently modified with these molecules was prepared by heatingunder reflux in toluene (Hong et al., 1987; Bae et al., 2000). In thefirst case the semiconductor surface becomes functionalized with amide(NH₂) groups (14, Scheme 1); in the latter with a chlorinatedhydrocarbon (15, Scheme 1).

Immobilization of Ferrocene Derivatives to Linker.

Four ferrocene derivatives, ferrocenylmethyl acetate (16) (Lindsay etal., 1957; Hayashi et al., 1980), ferrocenesulfonyl chloride (17)(Slocum & Achermann, 1982), 1-(hydroxymethyl)ferrocene (18) (Carlstrom &Frejd, 1990), and ferrocenylmethyl N,N′-dimethylethyldiamine (19)(Hayashi et al., 1980), were synthesized as described in the literature.The aminopropyl linker on TiO₂ was used to immobilize ferrocenylmethylacetate (Hayashi et al., 1980) and ferrocenesulfonyl chloride (Hong etal., 1987), while the chloropropyl linker was used to immobilize1-(hydroxymethyl)ferrocene (Bae et al., 2000), and ferrocenylmethylN,N′-dimethylethylenediamine (Johnson et al., 1999), as shown inScheme 1. In all cases, a color change of the TiO₂ powder from white toorange-brown indicated that the linking group on the TiO₂ reacted withthe ferrocene derivatives.

In three cases, the tether from the ferrocene group to the semiconductorsurface contains a secondary amine linkage (—C—NH—C—); in the fourthcase it contains an ether link (—C—O—C—). These contrast to the Si—C andC—C links that are present in the ODS-TiO₂ surface functionalization.Data obtained using the ferrocene derivatized TiO₂ semiconductormaterials indicate that ether and amine linkers are not photostable.Reaction with the photogenerated hole evidently cleaves these bonds,which means that the use of such linker is not practical. In contrast,linkers containing only carbon and hydrogens or Si—C bonds, particularlyalkyl linkers which are bonded to the semiconductor surface by —O—Si—bonds, are more stable. Ferrocenes can be preferably linked to thesemiconductor surface through with an alkyl chain. Compounds of theformula:(RO)₃Si(CH₂)_(n)Fcwherein the alkyl linker is the —(CH₂)n chain and R is any appropriategroup, particularly a small alkyl group or haloalkyl group, and ispreferably that the alkyl linker chain have 6 or more carbon atoms(e.g., that n is 6 or more), and more preferably that the alkyl linkerchain have 10 or more carbon atoms can reacted directly with thesemiconductor surface in a single hydrolysis step. Scheme 2 illustratesthe method.

The illustrated method can be employed for tethering of redox-activemoieties other than ferrocenes, including acridines and quiones, amongothers.

Additionally, substituted alkyl linkers that are not photoreactive canalso be employed, for example, linkers in which one, two, or more(including all) of the H of the CH₂ chain are replaced with F can beemployed and methods analogous to those of Scheme 2 can be used withsuch substituted linkers.

c. Co-Immobilization with Compensating Charged Groups

The charge of a redox-active species is often opposite than desired. Forextracting cations, for example, it would be optimal to have aswitchable negatively charged surface, but ferrocene/ferricenium goesfrom neutral to positive.

One way to change the effective sign of a surface is to co-immobilize acounterion which is not redox active. Tethering the appropriate numberof anionic groups such as sulfonate (—SO₃ ⁻), carboxylate (—CO₂ ⁻), orphosphonate (—PO₃ ⁼) along with ferrocene, for example, would yield asurface with net neutral charge when in the ferricenium form, and withnet negative charge when in the neutral ferrocene form.

Attaching a redox-active species like ferrocene and a compensatinganionic group pairwise to a surface can be done as illustrated instructures 21 and 22:

Both alkyl tails of a long-chain carboxylic anhydride (21) are attachedto the surface:

The anhydride linkage is then cleaved to attach ferrocene to one alkylchain, while leaving a carboxylic acid group on the other (22):

It is then straightforward to generate the carboxylate (CO₂⁻)-ferricenium (Fc⁺) ion pair. The carboxylate can be readily replacedwith sulfonate.

Several of the exemplified systems constitute prototypes in which theswitchable solute binding is driven by light, via irradiating of asemiconductor material In these cases, elution of an adsorbed specieswill occur merely by changing the illumination on the semiconductormaterial. No additional reagents are required, and no new wastewater isgenerated. Instead, the adsorbed solutes can be concentrated into asmaller volume of solution. Light can, for example, be used to drive aredox-active system by using a semiconductor as the substrate.Absorption of photons having energy greater than the semiconductor'sband gap generates electron-hole pairs, which are highly active“reagents” for causing redox reactions. The holes are powerful oxidizingagents, while the energetically promoted electrons are strong reducingagents.

d. Electrosorption by Redox-Active Surfaces.

Co-tethering a redox-active group and a compensating ion can be employedfor electrosorption. While electrosorption has seemed a promisingtechnique for water purification since the 1960s, but only recently haveimprovements in electrode materials made it appear practical (e.g.,Farmer et al., 1996). The technique is elegantly simple in conception:charged electrodes attract dissolved ions out of water. When theelectrodes are filled to capacity, they can then be regenerated merelyby switching their polarity. Thus electrosorption constitutes anelectrical approach to switchable absorption.

Although electrode surface areas and cycling lifetimes are muchimproved, electrosorption still suffers from problems with efficiency.One problem is leakage currents from the charged electrodes.Fundamentally, charging an electrosorbing electrode is analogous tocharging a capacitor, as was pointed out over 30 years ago (Johnson &Newman, 1971). In the ideal situation, current will flow at a decliningrate until the electrode is fully charged, at which point the currentflow will cease. Analogously, once charged a perfect capacitor willremain charged indefinitely without the application of further current.

This analysis, however, assumes a so-called “perfectly polarizableelectrode”, which is capable of maintaining an arbitrary potential at aninterface (in this case the water) without current flow across it. Suchelectrodes represent an unattainable ideal. Real electrodes do transfera certain amount of current across an aqueous interface, just as realcapacitors cannot hold a charge indefinitely. Such “leakage currents”are a significant source of inefficiency in electrosorption, becausemaintaining the electrodes at a constant potential requires an ongoingexpenditure of current. Moreover, the leakage currents worsenconsiderably as the water becomes more saline, because of the water'shigher conductivity, and with the result that electrosorption is atpresent impractical for purifying more concentrated brines such asseawater.

One way to lessen leakage inefficiency would be to place fixed chargeson the electrode surface. This is indeed how ion-exchange materialswork; they contain surface charges (e.g., sulfonate groups) that attractions of opposite charge from solution, and the charged groups on thesurface are not free to move into the solution. Of course, suchmaterials do not exhibit the switchable adsorption that is sopotentially valuable for electroabsorption.

The leakage currents can be significantly decreased while maintainingthe advantage of switchability by using redox-active electrode surfaces.Redox species present on the electrode surface provide a way to “latch”charge in place, with the result that leakage currents are greatlydiminished. In effect, fixed charges are generated in place by theapplied voltage. The electrode surface remains switchable, however,because reversing the potential reduces (or oxidizes) the redox speciesback to their original state. The only leakage losses occur during thecharging or discharging of the electrode surface; no current is expendedmerely in maintaining the electrode potential.

For example, such a “latching” electrosorption system can be based ontethered ferrocene. Ferrocene groups are tethered to a first electrode,for example, a high-surface-area porous carbon electrode, covalently viaalkyl chains or non-reactive substituted alkyl chains, so that theferrocene can oxidized to ferricenium (and rereduced to ferrocene) byapplication of the proper potential to the electrode. This generatesfixed positive charges on the electrode surface.

At the second electrode a ferricenium is co-tethered (as described aboveand illustrated in 21 and 22) with an equivalent amount of an anionicgroup, such as carboxylate (CO₂ ⁻) or sulfonate (SO₃ ⁻). This secondelectrode thus is also initially electrically neutral, because thepositive charge on every ferricenium is compensated by an adjacentanionic group.

Electro-oxidation of the first electrode to ferricenium, which leavesthat electrode positively charged, is accompanied by concomittantreduction of the ferricenium to ferrocene on the second electrode. Thisleaves the second electrode with a net negative charge, because thecharge on the anionic groups is no longer compensated. At this point thecharged groups on both electrodes can attract ions of compensatingcharge out of the solution, just as with the charged groups on anion-exchange resin as illustrated in FIG. 2. However, this systemremains switchable, just like a conventional electrosorption electrode:reversing the electrodes' polarities will reduce the ferricenium on thefirst electrode back to ferrocene while re-oxidizing the ferrocene onthe second electrode to ferricenium. At this point the extracted ionsare released.

Because the same redox species (ferrocene, is exemplified) is present oneach electrode, the energy cost of switching should be minimal.Oxidation of a ferrocene (or reduction of ferricenium) on one electrodeis simply compensated by a corresponding reduction or oxidation at theother electrode. Moreover, just as in conventional electrosorption, muchof the charging energy can be recovered on the discharge cycle, as thesystem acts like a high-capacity capacitor.

Redox-active electrode surfaces are known in the context of“pseudocapacitance”, in which reduction or oxidation of speciesincreases the effective charge capacity of the electrode. The phenomenonhas attracted attention in the context of “supercapacitors”, as itprovides a way to greatly increase electric storage capacity (e.g.,Conway et al., 1997). The phenomenon has not been proposed forapplication in electrosorption, however.

Other Applications: A material whose charge state can be “switched” byexternal triggers such as light, electricity, or chemical potential hasa great number of potential applications that fall into several broadcategories. As discussed above, switchable extraction, particularly viaphoto-driven switching, of solutes from solution (discussed above) is amajor application of such materials using, among others for, pollutioncontrol, water purification, desalination and hydrometallurgy. Inaddition, electrosorption by redox-active surfaces can employ suchmaterials.

Switchable redox materials, including those of this invention, can beapplied to the generation of reusable writing or data storage media forgeneral information science applications, for storage of light energy inchemical forms, as in “artificial photosynthesis” for making fuel fromsolar energy, generally for making electrochromic materials, whoseoptical properties (e.g., color, reflectivity, transparency) change onapplication of an electric field; and in electrocatalysis which useselectricity to drive chemical reactions, e.g. to carry out moreefficient syntheses of valuable products or to store electricity bymaking fuel.

The explosive growth of computer usage has hardly led to a “paperlesssociety”. Because of the inconvenience of reading off a monitor screen,as contrasted with the printed page, computer text files are commonlyprinted out even when the material to be read is of an extremelyephemeral nature such as e-mail messages or current news. Such paper isusually discarded almost immediately, putting additional pressure onboth disposal systems and forest resources. A printing medium that wasas convenient as paper to read, but that could be reused repeatedly,would be very attractive both from the standpoint of economics and fromthat of minimizing ecological “footprint”. “Paper” sheets that could beprinted off a computer, but that could be erased (say) by intense light,or by standing in the dark for a few hours, or by application of anelectric field, are obvious potential applications.

“Photochromic” materials change their color, or other opticalproperties, on exposure to light; “electrochromic” materials changethese properties on application of an electric field. Both have been thesubjects of intense interest for applications such as self-shadingwindows. Redox-activated surfaces suggest a new approach to theseapplications that may prove more practical.

“Artificial photosynthesis”, the use of sunlight to produce fuel, has aliterature stretching back to the early 1970s (e.g., Fukujima & Honda,1972; Bard & Fox, 1995; Bolton, 1996) although it is not yet practical.Photo-driven switchable redox systems of this invention in whichelectron-hole pairs generated in illuminated semiconductors driveenergetically “uphill” reactions, can be used to effectively store partof the energy of the photon into new chemical bonds. Several of thesystems illustrated herein are “photosynthetic” in that thephotoreduction of the oxidized species is energetically uphill. Hencesome of the energy of the absorbed photon has been trapped into achemical form. Furthermore, key issues in efficient photosynthesis by asemiconductor include efficient hole-pair separation, and minimumback-reaction between the chemical products formed. A recent paper(Gregg et al., 2001) has emphasized the importance of “kineticallyinhibited” redox couples for minimizing back-reaction in practicalphotosynthetic systems. A number of self-reversing systems describedherein exhibit slow but spontaneous reversal due to reaction withatmospheric oxygen after illumination is discontinued. In such cases thesluggishness of the back-reaction demonstrates a large kinetic barrierto reaction. In some cases a great difference in hydrophobicity betweenthe reduced and oxidized forms imposes the kinetic barrier. Applicationsof such kinetically inhibited systems to solar energy storage isstraightforward.

Switchable redox surfaces also are highly relevant to electrocatalysis,the use of electric potential to catalyze desired chemical reactions.They are particularly relevant to electrosynthesis, the use ofelectricity to drive chemical reactions in the thermodynamically uphilldirection. A particularly important application of electrosynthesis inthe coming years is likely to be the electrosynthesis of fuels, whichwould both furnish a way to store large amounts of electricity and tomaintain supplies of fuel.

Those of ordinary skill in the art will appreciate that materials andmethods other than those specifically exemplified herein are known inthe art and can be employed in the practice of this invention withoutrequiring undue experimentation. For example, redox-active species otherthan those specifically exemplified can be employed. Synthetic methodsother than those specifically exemplified can be employed. The methodsand materials of this invention can be applied for extraction and orseparation of charged species other than those specifically identified.References cited above and listed below are intended in part to providean overview of the state of the art and are incorporated by referenceherein to the extent that they are not inconsistent with the teachingsherein to provide additional description, illustration and examples ofmaterials and methods that are known in the art and that can be employedin the practice of this invention. TABLE 1 Photoswitching of ExemplaryQuinones Compound Solvent¹ Photoswitching² Comments³2-anilino-1,4-naphthoquinone(2) EtOH/H₂O 9:1 Bright orange

pale yellow Stays in solution EtOH/H₂O 2:8 Pale purple

purple Stays on surface of ODS-TiO₂ MeCN/H₂O 9:1 No Untreated TiO₂EtOH/H₂O 9:1 No 1,2-naphthoquinone-4-sulfonic acid, K salt EtOH/H₂O 1:9Yellow

colorless Stays in solution (4′-dimethylaminophenylimino)quinolin-8-one(3) EtOH/H₂O 1:9 (pH 12) Purple-blue

pale brown Stays on surface of ODS-TiO₂ EtOH/H₂O 1:9 (pH˜6) No1,2-naphthoquinone EtOH/H₂O 9:1 Yellow → colorless only Stays insolution (No reversal) 4-amino-1,2-naphthoquinone hemihydrate EtOH/H₂O9:1 No Stays in solution 1,4-benzoquinone EtOH/H₂O 1:9 No Stays insolution¹Volume ratios of solvent mixtures.²The change in color on illumination is noted; the colors are separatedby a double-headed arrow if photoswitching occurs (i.e., spontaneousreversal in the dark), a directed arrow i used if photoreduction occurswith no reversal.³All experiments conducted in the presence of ODS-TiO₂ unless “untreatedTiO₂” is noted; The phrase “stays in solution” indicates that thecompound (and its reduced form) do no absorb to the TiO₂

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1. A redox-switchable material which comprises: a solid semiconductor;and a redox-active moiety adsorbed, bonded or both to the surface of thesolid semiconductor.
 2. The material of claim 1 wherein the redox-activemoiety also comprises a selective complexing moiety.
 3. The material ofclaim 2 wherein the selective complexing moiety is a cryptand.
 4. Thematerial of claim 3 wherein the selective complexing moiety is a crownether.
 5. The material of any of claims 1 wherein the semiconductor ishydrophobic.
 6. The material of claims 5 wherein the redox-active moietyis covalently bonded to the surface of the hydrophobic semiconductor. 7.The material of claim 6 wherein the redox-active moiety is covalentlybonded to the surface of the hydrophobic semiconductor through a linkergroup.
 8. The material of claim 7 wherein the linker group is ahydrocarbon linker group.
 9. The material of any of claims 1 wherein theredox-active moiety comprises a ferrocene.
 10. The material of any ofclaims 1 wherein the redox-active moiety comprises an acridine.
 11. Thematerial of any of claims 1 wherein the redox-active material comprisesa quinone.
 12. The material of any of claims 1 wherein the hydrophobicsolid semiconductor comprises particles of semiconductor coated withsurfactant
 13. The material of claims 1 wherein the hydrophobic solidsemiconductor comprises particles of semiconductor covalently bonded tohydrophobic moieties.
 14. A reversible redox system which comprises aredox-switchable material of any of claims 1 in contact with a source ofelectrons.
 15. The reversible redox system of claim 14 also in contactwith an electron acceptor.
 16. The reversible redox system of claim 14further comprising a light source to irradiate the semiconductor togenerate electron-hole pairs.
 17. The reversible system of claim 14 incontact with an aqueous solution containing an oxidizing agent whereinthe redox-active moiety in its oxidized state can be photoreduced uponirradiation of the semiconductor and in its reduced state can beoxidized by the oxidizing agent.
 18. The system of claim 17 wherein theoxidizing agent is oxygen.
 19. The system of claim 18 wherein the oxygenis present during photoreduction of the redox-active moiety.
 20. Amethod for extraction of a selected solute from an aqueous solutionwhich comprises the steps of: (a) contacting the redox-switchablematerial of any of claims 1 with the aqueous solution, and thereafter(b) irradiating the semiconductor particles thereof to photoreduce theredox-active moiety of the material wherein solute is selectivelyadsorbed or complexed to the semiconductor on irradiation and whereinsolute is thereby extracted in whole or in part from the aqueoussolution.
 21. The method of claim 20 further comprising the steps of:(c) separating the irradiated semiconductor material and adsorbed solutefrom the aqueous solution, and thereafter (d) contacting thesemiconductor material and adsorbed or complexed solute with anoxidizing agent to oxidize the redox-active moiety and release adsorbedor complexed solute from the semiconductor material.
 22. The method ofclaim 19 further comprising repeating steps a, b, and c as many times asneeded to extract a desired amount of solute from the aqueous solution.23. The method of claim 19 wherein released solute is collected furthercomprising repeating steps a, b, c and d as many times as needed tocollect a desired amount of solute from the aqueous solution.
 24. Aphotoerasable writing media comprising the redox-switchable material ofany of claims
 1. 25. A electrochromic or photochromic material whichcomprises the redox-switchable material of any of claims
 1. 26. Acatalyst which comprises the redox-switchable material of any of claims1.
 27. A method for storage of light energy which comprises the step ofemploying a redox-switchable material which comprises a redox-activemoiety adsorbed, bonded or both to the surface of a photoactive solidsemiconductor to mediate or catalyze a chemical reaction.
 28. The methodof claim 27 wherein the redox switchable moiety is selected from thegroup consisting of a cryptand and a crown ether.